This invention relates to a view finder system for a single-lens-reflex-camera of the type in which an image of the indicator mounted in the lens barrel which is connected to the camera body is provided in the field of the finder.
The presently proposed devices for providing views or images of various photographic informations in or outside the photographing image in the field of finders for photographic cameras may be considered to be divided into two main groups, one of which is composed mainly of mechanical means, the other is composed mainly of optical means. As far as the simplicity of structure is concerned, the latter is superior to the former. View finder systems of the later type and particularly adapted for use in single lens reflex cameras include a pentagonal roof prism in combination with supplementary optical means optically connected thereto by means of which an image of at least part of the photographic information indicator such as a distance adjusting ring, diaphragm adjusting ring and the like movably mounted in the lens barrel connected to the camera body is provided in the finder field of view. Such view finder systems are described, for example, in the German Pat. No. 1,196,493 and U.S. Pat. No. 3,014,911.
The view finder system of the German Pat. No. 1,196,493 issued March 31, 1966 includes a pentagonal roof prism in combination with supplementary optical means which comprise a pivoted mirror element arranged in front of the front reflecting face of the pentagonal roof prism, and a prism having a reflecting face and arranged under said pivotably mirror element in alignment with said mirror and the scales carried on the lens barrel of the camera. Upon tilting of the mirror element at a predetermined angle, either of a scale carried on the lens barrel and a scale of an indicator mounted above the mirror element is selected to provide an image thereof in the finder image. The light rays reflected from the mirror element are allowed to directly enter the pentagonal roof prism at a half-mirrored portion of the front reflecting face which contributes to the formation of the finder image. As a result, the finder image of the view finder is darker in a portion corresponding to the half-mirrored area on the front reflecting face. Another disadvantage is that, because of a large angle which the reflecting face of the supplementary prism makes with respect to the horizontal, a considerable amount of light rays reflected from the area surrounding the scale are allowed to enter the view finder, causing the reading of the scale image to become difficult. In order to overcome the latter disadvantage, according to the identified patent, instead of the supplementary prism, a convex lens is employed as positioned directly above the scale carried on the lens barrel, thereby light rays from the scale are collected by the convex lens and the allowed to impinge on the mirror element. However, in this case, the finder housing extends largely toward the lens barrel side to create an alternative difficulty for the camera operator in viewing the selected indicium of the scale directly from outside.
The view finder described in the U.S. Pat. No. 3,094,911 issued June 25, 1963 is constructed as comprising a roof pentaprism provided with a sub-prism cemented either to the upper front face, or to the lower front face which contributes the formation of the finder image. The sub-prism has two reflecting faces disposed in such a manner that a beam of light rays from the indicia of the exposure condition adjusting components movably mounted in the lens barrel is reflected by one of the two to the other, and therefrom reflected to the interface between the sub-prism and the pentaprism. The beam entering the interface is directed through the pentaprism to the eye of the observer looking through the view finder. In this arrangement of the optical elements of the view finder, it is necessary to dispose the first and second reflecting faces of the subprism with high accuracy not only relative to each other, but also with respect to the roof pentaprism, or other wise the indicium image will not appear within a predetermined field of view relative to the finder field of view. In manufacturing sub-prisms having the specified surfaces and assembling a sub-prism with a roof pentaprism, therefore, the requirements for accuracy are rigorous in view of the incapability of, once the sub-prism has been cemented to the roof pentaprism, modifying the spatial and angular position of the first and second reflecting faces of the sub-prism. Further, in order to provide an image of portions of the indicia distributed over an increased area, it is necessary to increase the area of the first reflecting face of the sub-prism and consequently increase its volume, thereby preventing the size and weight of the view finder from being minimized, as long as the first reflecting face is arranged not to be movable.
The finder system described in the U.S. Pat. No. 3,800,655 applied on Dec. 14, 1972 and published on Apr. 2, 1974 has a pentagonal roof prism in which a V-shaped seat is provided between a front reflecting face and a front non-reflecting face, a sub-prism cemented to said V-shaped seat, and a mirror which optically faces said sub-prism and an indicator provided at a lens barrel. The light beam from the indicator is first reflected by the mirror and is directed to the sub-prism. The luminous flux from the indicator passing through the sub-prism is so reflected by the front reflecting face of the pentagonal roof prism as being directed to almost same direction as that of the luminous flux from a focusing screen. Therefore, it is necessary to provide a V-shaped seat through which said luminous flux passes at the pentagonal roof prism so that the luminous flux from the indicator impinges on the front reflecting face from almost same direction as that of the luminous flux from the focusing screen. Further, two reflecting planes, first and second, which face up or down respectively, need to be provided at the sub-prism for having the luminous flux from the indicator pass through the V-shaped seat along the above mentioned direction, also for increasing the optical path of said luminous flux. The first reflecting plane faces the above mentioned mirror while the second reflecting plane faces the first reflecting face of the pentagonal roof prism. And the first and second reflecting planes face to each other.
The greatest shortcoming of the finder system of U.S. Pat. No. 3,800,655 is complication of the shape of two prisms. The pentagonal roof prism has a V-shaped seat difficult to manufacture, also the sub-prism has a complicated shape and a large size. Therefore, manufacturing cost of such finder system is high.